With regard to nonvolatile semiconductor memory devices such as flash memory or the like, memory cells of various structures are being developed in order to have higher integration. In particular, attention is being focused on technology for raising the integration level of the memory cells by having the memory cells in a three-dimensional structure.
Patent Document 1 discloses a nonvolatile semiconductor memory device in which a columnar protrusion is provided on a surface of a semiconductor substrate and a charge accumulating film (or layer) is provided on a sidewall of the columnar protrusion. FIG. 8 shows a cross-sectional view thereof. As shown in FIG. 8, the charge accumulating film 124 is formed on the sidewall of the columnar protrusion 102 provided on the surface of the semiconductor substrate 101, and a control gate 108 is formed so as to sandwich the charge accumulating film 124 with the columnar protrusion 102. The charge accumulating film 124 and the control gate 108 are formed so as to surround at least a part of the periphery of the columnar protrusion. That is, in Patent Document 1, by forming the charge accumulating film 124 and the control gate 108 on the sidewall of the columnar protrusion 102 provided on the surface of the semiconductor substrate 101, it is possible to ensure an adequately large capacity between the charge accumulating film 124 and the control gate 108, with a small occupying area. Furthermore, by a drain 110 being split by a groove, an element separation region can be made smaller. In Patent Document 1, one memory cell is formed for each columnar protrusion 102.